1. Field of the Invention
The present invention relates to a distance measuring system that measures a distance based on the propagation time of a pulse signal from a transmitter to a receiver.
2. Description of the Related Art
Conventionally, there has been proposed a distance measuring system which uses radio waves as a so-called distance measuring method to measure a distance between two different points.
To measure a distance l from a point F to a point G, for example, a transmitter 61 and a receiver 62 as radio stations are respectively placed at the point F and the point G in the distance measuring system as shown in FIG. 9. The transmitter 61 transmits a radio wave comprised of pulse signals, which are received by the receiver 62. A propagation time Td for the pulse signal transmitted from the transmitter 61 to be received by the receiver 62 is measured. The distance l can be calculated by multiplying the measured propagation time Td by a propagation speed Vc of the radio wave. As the propagation speed Vc of the radio wave is constant, the distance l can be measured accurately by measuring the propagation time Td alone.
Proposed methods of measuring the propagation time Td include a method of employing a spread spectrum technology which measures a distance between transceivers at the phase timing of a spread code in transmission and reception in addition to a method of measuring the time at which the pulse signal zero-crosses, and a method of measuring the propagation time Td by identifying a phase difference of the pulse signal.
FIG. 10 shows a conventional system configuration using the spread spectrum technology. The system measures a distance d between a transceiver 71 and a transceiver 72 located at two different points. The transceiver 71 includes a spread-signal generator 81 which generates a baseband spread code, a transmitting unit 82 connected to the spread-signal generator 81, an antenna 83 connected to the transmitting unit 82, an antenna 84 for receiving a radio wave from the transceiver 72, a receiving unit 85 connected to the antenna 84, a correlation calculator 86 connected to the receiving unit 85, a correlation-position determining unit 87 connected to the correlation calculator 86, and a distance measuring unit 88 connected to the spread-signal generator 81 and the correlation-position determining unit 87. The distance measuring unit 88 finally measures the distance d.
The transceiver 72 includes an antenna 91 for receiving a radio wave sent from the antenna 83 of the transceiver 71, a receiving unit 92 connected to the antenna 91, a frequency converter 93 connected to the receiving unit 92, a transmitting unit 94 connected to the frequency converter 93, and an antenna 95, connected to the transmitting unit 94, for transmitting a radio wave.
The spread-signal generator 81 in the transceiver 71 generates a baseband spread code and a phase timing signal for the spread code. The transmitting unit 82 converts the generated spread code to a high-frequency signal with a center frequency f1, and transmits the high-frequency signal to the transceiver 72 via the antenna 83.
In the transceiver 72, the receiving unit 92 amplifies the high-frequency spread code received via the antenna 91, the frequency converter 93 converts the frequency of the amplified spread code to a center frequency f2, and the resultant code is retransmitted to the transceiver 71 via the transmitting unit 94 and the antenna 95. The transceiver 71 receives the retransmitted spread code via the antenna 84 and the receiving unit 85, and converts the high-frequency spread code to a baseband spread code by orthogonal detection. The correlation calculator 86 performs autocorrelation on the spread code, and the correlation-position determining unit 87 detects the phase timing of the received spread code based on an autocorrelation peak position. The distance measuring unit 88 detects a difference T1 between the phase timing of the transmitted spread code and the phase timing of the received spread code, and calculates the distance d between the transceivers 71 and 72.
Non-patent Literature 1
    J. Lampe, R. Hach, L. Menzer IEEE-15-05-0002-00-004a January 2005
Recently has been proposed a distance measuring system that generates a pulse sequence having a plurality of pulse signals of equal amplitudes arranged at equi-time intervals in a transmitter, transmits a radio wave comprised of the pulse sequence to a receiver from the transmitter, and calculates, in the receiver, the distance from the propagation time of each of the pulse signals in the pulse sequence, as shown in FIG. 11. (See, for example, Non-patent literature 1.)
The distance measuring system is similar to the aforementioned distance measuring system in that the propagation time Td is acquired based on the time at which each of the pulse signals in the pulse sequence zero-crosses. For instance, a pulse signal J1 generated at time t51 in the transmitter in the example shown in FIG. 11 is received as a pulse signal J1′ by the receiver at time t61. The time interval from time t51 to time t61 is equivalent to a propagation time Td1. In the distance measuring system, not only the propagation time Td of a single pulse signal, but also the propagation times Td of the pulse signals J1′, J2′, J3′, J1′, Ji+1′, . . . received by the receiver are respectively acquired with respect to the pulse signals J1, J2, J3, Ji, Ji+1, . . . , which constitute the pulse sequence.
As a result, a propagation time Td2 calculated for the pulse signal J2′ with respect to the pulse signal J2, a propagation time Td3 calculated for the pulse signal J3′ with respect to the pulse signal J3, a propagation time Tdi calculated for the pulse signal Ji′ with respect to the pulse signal Ji, and so forth are sequentially acquired. The average value of the propagation times Td1, Td2, Td3, . . . , Tdi is calculated, and the distance from the transmitter to the receiver is acquired based on the average propagation time. The use of the average propagation time for the pulse signals in calculating the distance can suppress an error in measuring a distance of 30 m to about 81.7 cm.
Instead of acquiring the propagation time Td, the average propagation time may be acquired based on time differences between the pulse signals J1′, J2′, J3′, Ji′, Ji+′, . . . in the pulse sequence received by the receiver. Given that the pulse signals are formed at equi-time intervals T therebetween on the transmitter side as shown in FIG. 11, the time differences between the pulse signals J1′, J2′, J3′, Ji′, Ji+1′, . . . are expressed by T+d where d is an error difference of the propagation time T based on the influence of another radio wave, noise or the like over the time to reach the receiver from the transmitter.
For example, the time difference between the pulse signals J1′ and J2′ is expressed by T+d1, the time difference between the pulse signals J2′ and J3′ is expressed by T+d2, and the time difference between the pulse signals Ji′ and Ji+1′ is expressed by T+di (|di|>0). Acquisition of the average value of d1, d2, . . . , di makes it possible to calculate the average propagation time.
Instead of acquiring the average propagation time, time differences T+d1, T+d2, . . . , T+di between the pulse signals J1′, J2′, J3′, Ji′, Ji+1′, . . . in the pulse sequence received by the receiver may be calculated in the aforementioned manner, a pulse signal J′ which minimizes d in the calculated time differences T+d is specified, and the propagation time of the specified pulse signal J′ may be acquired. It is still possible to reduce an error in the distance to be actually measured in this case.
Even when the distance is calculated based on the average value of the propagation times of pulse signals in a pulse sequence according to the conventional method, however, an error of about 80 cm is included in a distance of 30 m after all. This does not make it possible to provide a system configuration adaptable to a location where a high-precision distance measurement with a smaller error is desired, so that the versatility of the overall system cannot be enhanced, disadvantageously.